The use of a sun shield would help prevent a runaway greenhouse event. To allow a runaway event is suicide. Our species can use technology to control the weather and promote healthy ecological niches for our current life forms thus preventing an extinction event. The sun shield would be a part of a larger many faceted program of converting to sustainable energy, reducing machine and per capita energy usage, cleaning up the oceans, eliminating pollution, and creating 100% recycling. This many faceted approach will retain life in every environmental niche so it can live how it lives now.
The effect of the sun shield on earth is to redirect 2% of the sunlight away. It would not to create a shadow like an eclipse. There is no impact ecologically from this 2% since all life on Earth can handle a cloudy day which is about a 20-40% reduction in sunlight and all life on Earth handles nighttime which is a 100% reduction in sunlight. Thus no threat exists to the environment. What the 2% does is stop the warming and as we reduce greenhouse gas emissions we can cool back down a little bit to the weather of last century which was better for crop production.
The cost to build is negligible over the 50 year project life because the amount of money needed is about two thousandths of the current world Gross Domestic Product, that is, all the money that gets spent in a year worldwide. In hard cash we need 850 billion up front and 4 trillion over the 50 years in operations. This is roughly the same amount of spending as the 5 trillion America spent during the last 15 years of military actions. We could do it ourselves if we wanted because we have enough money to afford it.
If we do nothing we are faced with extinction, the permanence of death, and I think that not wanting to spend money to stay alive is at best irrational and at worst immoral. We should spend every dime we need to spend for future generations to have a better environment than we have now.
Everything on Earth has evolved to survive in the existing environment. Placing a sun shield in space would drastically change that environment and cause unknown amounts of harm to life of all kinds everywhere. Humans have already done enough environmental damage to this planet, and our future is already at risk from the existing harm. To increase this intentionally by creating a sudden change in the environment makes no sense - we need to focus on protecting and repairing the planet.
A space-based sun-shield is an ambitious project and would be costly. Considering the current world economy, I doubt that any government would support funding such a project. I believe the money would be better spent on tax incentives, investment in new technology and other projects to slow global warming on earth. Education and support is the key to battling climate change before it becomes too late.